Login as root;
  
  Update your APT database with apt-get update;
  
  Install the latest security updates with apt-get upgrade;
  
  Install required packages with apt-get install build-essential module-assistant;
  
  Configure your system for building kernel modules by running m-a prepare;
  
  Add virtualbox source in /etc/apt/sources.list
  
  apt-get install virtualbox-guest-x11
  
  Click on Install Guest Additions… from the Devices menu, then run mount /media/cdrom.
  
  Run sh /media/cdrom/VBoxLinuxAdditions.run, and follow the instructions on screen.